Normally, when biometric authentication is performed by checking finger veins, a user puts his/her finger on an area surrounded by the guides of an authentication apparatus (transparent plate of plastic or similar material) and the finger is irradiated with light of a prescribed wavelength from light sources provided on the guides and an image of the finger thus irradiated is taken by a camera, etc. The vein pattern of the finger is read with a scanner, etc. to obtain an image of the vein pattern. Then, whether or not the image of the read vein pattern coincides with a previously registered vein pattern is checked for personal authentication.
When the above method is used for personal authentication, it is desirable that external light including natural light such as sunlight and ambient light such as light from lamps, and light from the light sources which impinges on the finger, should not enter the area surrounded by the guides. The reason is that such light is reflected by the transparent plate and the reflected light may show up in a camera image of the user's finger, causing the camera image to be blurred and resulting in deterioration in personal authentication accuracy.
As a technique for eliminating the possibility of unwanted light showing up in a camera image, JP-A No. 2005-128935 discloses an authentication apparatus which uses a lid to cover the user's finger to block external light, etc. However, according to the technique described in JP-A No. 2005-128935, the lid is so shaped as to cover the user's finger entirely, which may make the user nervous and psychologically stress the user.
As a technique for easing the user's psychological stress, JP-A No. 2003-187235 discloses an authentication apparatus which uses guides for surrounding only the lateral faces of the finger instead of the above lid in order to block external light coming from both the lateral faces of the finger and reduce the possibility of reflected light showing up in a camera image while reducing the user's psychological stress.